The present invention relates to lead germanium oxide (PGO) thin film materials, and particularly to the spin-coating of PGO ferroelectric thin films in one-transistor applications.
Ferroelectric films have attracted great interest in recent years because of their applications in electro-optic, pyroelectric, frequency agile electronic and non-volatile memory devices. The fabrication and characterization of ferroelectric lead germanium oxide thin films (PGO), such as Pb5Ge3O11 and Pb3GeO5, are of current interest. Lead germanite (Pb5Ge3O11) is a relative new member of ferroelectric family. The piezoelectric, dielectric and electric-optic properties of single crystal and polycrystalline materials have been reported in the literature. Pb5Ge3O11 is an optically active and ferroelectric material, has a moderate dielectric constant and a small remanent polarization, which make it particularly suitable for ferroelectric non-volatile memory devices such as metal ferroelectric metal oxide silicon (MFMOS), metal ferroelectric metal silicon (MFMS), metal ferroelectric insulators silicon (MFIS), metal-insulator-ferroelectric-silicon (MIPS), metal-insulator ferroelectric insulators silicon (MIFIS), and metal-ferroelectric-silicon (MFS) type memories. Pb5Ge3O11 also has potential in thermal detector applications because of its pyroelectric and dielectric characteristics. Pb3GeO5 is a ferroelastic material, which may be used for microelectromechanical systems (MEMS) applications.
Chemical vapor deposition (CVD) is a particularly attractive method for semiconductor industries because it is readily scaled up to production runs and provides very good step coverage. For PGO (Pb5Ge3O11 and Pb3GeO5) films, the content of lead to germanium is very high, i.e., 5:3 and 3:1, respectively. Because of Pb loss in the thermal MOCVD process, an excess Pb concentration and a high oxygen partial pressure must be used to make stoichiometric PGO thin films and avoid the Pb and O deficiency.
The Pb precursors for MOCVD PGO film are liquids or solids that can be sublimed into a gas phase and transported into a reactor. The process window of Pb precursors is very narrow, i.e., the sublimation temperature of the Pb precursors is close to the decomposition and condensation temperature, in which case the reagent may begin to decompose or condense in the reactant lines before reaching the reactor, making it very difficult to control the stoichiometry of the deposited films. The Pb precursor gas easily reacts with oxygen in the gas phase before deposition, which results in large particles and a cloudy film, especially at higher deposition temperatures.
Because of some of the problems associated with MOCVD and known spin coating processes of PGO thin films, other solutions and techniques may be more suitable for deposition of this material in integrated circuit devices by spin coating. There has been some discussion of PGO material suitable for spin coating and of the spin coating process in Kim et al., Japanese Journal of Applied Physics 33, pp 2675-2678, 1994; and Lee et al., Applied Physics Letters 60, pp 2487-2488, May 18, 1992.
Known techniques for spin coating PGO films are similar to those used for PZT ferroelectric thin films wherein Pb(OCH3CO)2.3H2O is used as the Pb source and Ge(OCH2CH3)4 is used as the Ge source. The Ge source, however, is air and moisture sensitive, and as the Pb source contains water, a PGO solution formed in this manner will be unstable, and a Ge gel will generally precipitate from the solution.
A method of preparing a PGO solution for spin coating includes preparing a 2-methoxyethanol organic solvent; adding Pb(OCH3CO)2.3H2O to the organic solvent at ambient temperature and pressure in a nitrogen-filled gloved box to form Pb in methoxyethanol; refluxing the solution in a nitrogen atmosphere at 150xc2x0 C. for at least two hours; fractionally distilling the refluxed solution at approximately 150xc2x0 C. to remove all of the water from the solution; cooling the solution to room temperature; determining the Pb concentration of the solution; adding the 2-methoxyethanol solution to the Pb 2-methoxyethanol until a desired Pb concentration is achieved; combining Ge(OR)4, where R is taken from the group of Rs consisting of CH2CH3 and CH(CH3)2, and 2-methoxyethanol; and adding Ge(OR)4 2-methoxyethanol to PbO 2-methoxyethanol to form the PGO solution having a predetermined metal ion concentration and a predetermined Pb:Ge molar ratio.
An object of the invention is to provide a PGO solution suitable for spin coating application.
Another object of the invention is to provide a techniques for spin coating a PGO film in a one-transistor application.
This summary and objectives of the invention are provided to enable quick comprehension of the nature of the invention. A more thorough understanding of the invention may be obtained by reference to the following detailed description of the preferred embodiment of the invention in connection with the drawings.